Bioremediation is an established technology for removing PAHs from contaminated soil, but previous studies have shown that it does not always lead to a reduction in toxicity. The causes of toxicity and the mechanisms by which toxicity might be avoided or diminished are not well-understood. We hypothesize that metabolites produced by PAH-degrading bacteria, which have been observed to accumulate in field-contaminated soil and sediment, are responsible at least in part for the toxicity that can result from bioremediation. We also hypothesize that bioremediation conditions influence the community of PAH-degrading microorganisms in contaminated soil, which in turn influences both PAH removal and the extent to which metabolites might accumulate. We propose to explore the effects of bioremediation conditions on PAH removal and soil toxicity, using a slurry-phase bioreactor as the experimental platform. Among the conditions we will evaluate is the addition of a hydrophobic surfactant at a low dose, which we recently demonstrated can improve the bioavailability and biodegradation of PAHs that remained in a field-contaminated soil after conventional bioremediation. This approach will be developed further by demonstrating its efficacy in a semi-continuous process. We will combine our recent work on stable-isotope probing with a high-throughput DNA sequencing method to identify the PAH-degrading bacteria most likely to influence PAH removal, metabolite accumulation, and toxicity in the treated soil. Genomes of these organisms will be sequenced to identify genes associated with PAH metabolism, and the key genes will be expressed. Differences in the ability to metabolize various PAHs will be correlated to sequence differences in these genes so that genetic determinants of metabolite accumulation can be identified. In parallel, we will use fractionation techniques and advanced analytical tools to identify compounds responsible for toxicity of the treated soil. Finally, variables that can be controlled during bioremediation will be evaluated for their ability to preclude or mitigate toxicity. Our overall goal is to fill key gaps in knowledge that will inform and improve field applications of bioremediation that lead to true reductions in risk.